Normalize null ConstructionStockpile.Items before raising a paste

ClientPlugin/Patches/NullSafety/MyPasteGridParametersPatch.cs

@@ -0,0 +1,74 @@
+using System.Collections.Generic;
+using HarmonyLib;
+using Sandbox.Game.Entities;
+using VRage.Game;
+using VRageMath;
+
+namespace ClientPlugin.Patches.NullSafety;
+
+// A blueprint saved from a world with a partially built block carries a
+// <ConstructionStockpile> with no Items children, which the game's XML reader
+// deserializes to Items == null, discarding the field's own empty-array
+// initializer. VRage's event serializer refuses that null when
+// MyCubeGrid.TryPasteGrid_Implementation is raised in a hosted session
+// (OnlineMode other than OFFLINE), so the paste fails:
+//   Error serializing MyObjectBuilder_ConstructionStockpile.Items,
+//   member contains null, but it's not allowed ...
+// This is a vanilla game bug: it reproduces on the Windows binaries under
+// Proton with this plugin absent. Every paste route funnels through this one
+// constructor, so normalizing here fixes the UI clipboard, the grid storage
+// helper, visual scripting and plugin-raised pastes alike. An empty Items
+// array round-trips as zero items, identical to what a stockpile with no
+// components means, so the wire format is unchanged.
+[HarmonyPatch(
+    typeof(MyCubeGrid.MyPasteGridParameters),
+    MethodType.Constructor,
+    new[]
+    {
+        typeof(List<MyObjectBuilder_CubeGrid>),
+        typeof(bool),
+        typeof(Vector3),
+        typeof(bool),
+        typeof(MyCubeGrid.RelativeOffset),
+        typeof(List<ulong>),
+    }
+)]
+[HarmonyPatchCategory("Init")]
+static class MyPasteGridParametersCtorPatch
+{
+    static void Prefix(List<MyObjectBuilder_CubeGrid> entities)
+    {
+        if (entities == null)
+            return;
+
+        foreach (var grid in entities)
+            NormalizeStockpiles(grid, depth: 0);
+    }
+
+    private static void NormalizeStockpiles(MyObjectBuilder_CubeGrid grid, int depth)
+    {
+        // Projected grids nest further projectors; bound the recursion anyway.
+        if (grid?.CubeBlocks == null || depth > 8)
+            return;
+
+        foreach (var block in grid.CubeBlocks)
+        {
+            if (block == null)
+                continue;
+
+            var stockpile = block.ConstructionStockpile;
+            if (stockpile != null && stockpile.Items == null)
+                stockpile.Items = [];
+
+            if (block is MyObjectBuilder_ProjectorBase projector)
+            {
+                NormalizeStockpiles(projector.ProjectedGrid, depth + 1);
+                if (projector.ProjectedGrids != null)
+                {
+                    foreach (var projectedGrid in projector.ProjectedGrids)
+                        NormalizeStockpiles(projectedGrid, depth + 1);
+                }
+            }
+        }
+    }
+}